Spinal cord injury (SCI) affects thousands of individuals a year in the United States resulting in a loss of voluntary control of muscle, severe muscle atrophy and bone loss. It has been observed that six weeks after SCI, skeletal muscles below the level of the lesion decrease in cross-sectional area and begin a shift toward a fast fiber phenotype. Individuals with SCI will typically lose 50-60% of their bone mineral density (BMD) because of deterioration of the trabecular epiphyses and a thinning of the cortical wall. Owing to this rapid musculoskeletal deterioration, individuals with SCI have a 2% chance of sustaining a lower extremity fracture, which is double the fracture risk of the general population.
Neuromuscular electrical stimulation (NMES) is a method for neurological rehabilitation in patients with, among other things, SCI. NMES generates contractions by depolarising axons under stimulating electrodes placed on the skin over a muscle belly or peripheral nerve trunk. Traditionally, NMES is delivered using relatively narrow pulse widths (0.05-0.4 ms) and low frequencies (20-40 Hz). This traditional type of NMES favors the activation of motor axons and, thus, generates contractions predominantly through a peripheral pathway that does not involve the central nervous system (CNS). Accordingly, traditional NMES recruits motor units in a non-physiological manner, with a random recruitment order of motor unit types and all motor units discharge synchronously.
Traditional NMES, however, has a number of disadvantages. The random recruitment order leaves fatigue-resistant muscle fibers, those most vulnerable to developing disuse-related complications after paralysis, relatively inactive. The synchronous discharge means that all motor units discharge at the same time and, thus, discharge rates must be abnormally high to produce smooth contractions of sufficient amplitude to produce functionally meaningful contractions. Such high discharge rates increase the metabolic demand on individual motor units compared to voluntary contractions of similar amplitude. Both of these non-physiological aspects of motor unit recruitment during NMES (random recruitment order, synchronous discharge) limit the benefits and widespread use of NMES for rehabilitation.
Additionally, NMES is delivered over at least one muscle at a time, typically while participants are supine. Such a traditional NMES approach results in, for example, contractions for thighs or lower limbs rather than multi-muscle stimulation of both limbs. Further, the contractions are also unlike voluntary contractions that are produced by output from the central nervous system, thus, potentially rendering an unfavorable plateau in muscle hypertrophy. Even still, low-intensity NMES-cycling has no effect on BMD for individuals with acute or chronic SCI. High-intensity NMES-cycling, by contrast, was shown to increase BMD by approximately 10-14% at the distal femur and proximal tibia for individuals with a chronic SCI.
Thus, there exists a need in the art for NMES of multiple muscle groups, involving the central nervous system and utilizing high-intensity NMES for greater rehabilitative efficacy in patients suffering from a neuromuscular injury, disease or disorder that results in paralyses. In addition, the NMES will be combined with functional tasks such as trunk extension and the sit to stand activities.